1. Field Of The Invention
This invention relates to novel processes for removing gaseous sulfur compounds including hydrogen sulfide from gas streams containing same to recover sulfur and to render said gas streams more environmentally acceptable and less harmful to catalysts used in subsequent operation on said gas streams. The field of this invention includes those industrial processes employed in the manufacture of synthesis gas for synthetic liquid or gaseous fuels and other chemicals.
2. Description Of The Prior Art
Synthesis gas is generally produced by the controlled oxidation of gaseous, liquid or solid fuels such as natural gas, petroleum or coal. It is essential to remove sulfur which is predominantly in the form of hydrogen sulfide prior to further treatment of the gases in order to avoid contamination or poisoning of catalysts used in such further treatments and in order to provide a more environmentally acceptable product. Sulfur removal is frequently achieved by treating the gas with a suitable absorbent solution, for example, of strong organic bases, which absorb H.sub.2 S and CO.sub.2 and in some cases other gaseous sulfur compounds such as COS and CS.sub.2. The loaded absorbent solution is stripped by heat, releasing an acid gas containing H.sub.2 S, CO.sub.2 and the other gaseous sulfur compounds, if present. The acid gas, in some cases, is disposed of by incineration producing combustion gases high in SO.sub.2 content and thus environmentally unacceptable. In other cases, they are subjected to the Claus reaction to recover some of the sulfur; but this procedure is expensive and/or also results in tail gases containing amounts of H.sub.2 S and SO.sub.2 which render them environmentally unacceptable.
U.S. Pat. No. 3,896,215 discloses a process for removing hydrogen sulfide from a gas stream containing it by reacting it with SO.sub.2 in a Claus reaction, afterburning the effluent gas to convert the sulfur content to sulfur dioxide and sulfur trioxide, reducing the sulfur trioxide content to sulfur dioxide, thereafter absorbing the sulfur dioxide content in an absorbent such as disodium hydrogen phosphate and desorbing the sulfur dioxide and returning it to the Claus reaction. The process of this patent makes no provision for the removal of heat stable salts formed in the absorption-desorption cycle used to remove sulfur dioxide and the disodium hydrogen phosphate becomes loaded with heat stable salts of sulfur oxyanions and thus becomes less effective in recovering sulfur dioxide. The build-up of heat stable salts in the absorbent, in fact, is considerably encouraged by the use of high temperatures in the regeneration (stripping) tower pursuant to the teachings of this patent. As a consequence, less and less sulfur dioxide is recycled to the Claus process and the system steadily becomes more unbalanced requiring constant adjustments to the inputs and operating conditions in the various units in the system and ultimately complete loss of effectiveness of the SO.sub.2 -absorbent. The patent also fails to provide means for handling gas streams having excessive amounts of H.sub.2 S without enlarging or overloading the Claus reactor.
There are a number of prior art methods for recovering sulfur oxides from gases containing them by absorbing and/or reacting the sulfur oxides with inorganic reagents, e.g., sodium carbonate, sodium hydroxide, ammonium hydroxide, aqueous ammonia, other alkali metal or alkaline earth metal hydroxides or carbonates and the like, in solution, slurry or powder form to yield the corresponding sulfate and sulfite salts (see, for example, U.S Pat. No. 1,908,731). In many of these processes, the absorbing solutions are regenerated by heating, in a separate vessel, thus liberating concentrated SO.sub.2. This desorption step does not, however, remove any sulfate, thiosulfates or polythionates that result from absorption of the sulfur trioxide and thermal disproportionation of sulfite and bisulfite and which eventually build up in the system.
In many of these cases, the reagent cannot be readily regenerated without the expenditure of considerable amounts of energy or considerable amount of other reagents. In those instances where a regenerated absorbent can be used, the sulfate concentration in the absorbent builds up both by absorption of sulfur trioxide or sulfuric acid mist which might be and usually are present in the stack gas and by oxidation of dissolved sulfur dioxide by the reaction of oxygen which is also sometimes present in the gas contacted with the absorbent. A further source of the build up of sulfates and/or other sulfur oxyanions of heat stable salts is disproportionation of dissolved sulfites and bisulfites in contact with dissolved sulfur dioxide. Such heat stable salts include, in addition to the sulfates, SO.sub.4.sup..dbd. ; the thiosulfates, S.sub.2 O.sub.3.sup..dbd. ; the dithionates, S.sub.2 O.sub.6.sup..dbd. ; the trithionates, S.sub.3 O.sub.6.sup..dbd. ; and other higher polythionates, S.sub.x O.sub.6.sup..dbd., and other divalent sulfur oxyanion-containing heat stable salts. The sulfates usually can be removed essentially quantitatively through the use of an alkali metal hydroxide equivalent to twice the molar concentration of the sulfate resulting in substantially quantitative precipitation of the sulfate as the di-alkali metal salt without precipitation of sulfite or bisulfite ions. However, the other divalent sulfur oxyanions of strong acids such as the thiosulfates, dithionates and higher polythionates also build up in the system and cannot be quantitatively removed by means of alkali metal hydroxide precipitation. Furthermore, the presence of such other divalent sulfur oxyanions of heat stable salts actively interferes with the quantitative removal of the sulfates.
In some instances, as in U.S. Pat. No. 3,503,185, the gas was prewashed to remove sulfates which were then purged from the system. In this type of system, as with the use of coke in U.S. Pat. No. 3,896,215, it is sought to eliminate sulfur trioxide content of the gas before contact with the SO.sub.2 absorbent. Such prewashes were not capable of removing all sulfur trioxide as sulfate and, of course, would not remove sulfates formed in other parts of the SO.sub.2 -recovery system. This patent, furthermore, does not disclose any means for eliminating the thiosulfates, dithionates and higher polythionates. U.S. Pat. No. 3,790,660 is similar in showing a water prewash to remove sulfur trioxide and fly ash. It specifies a sulfate purge stream to remove the sulfate; unfortunately, a considerable amount of the alkali metal sulfite and bisulfite also accompany the sulfate. This requires a considerable addition of alkali metal hydroxide to make up for the loss. Furthermore, there is no system disclosed for removing the thiosulfates, dithionates or other polythionates except by purging them with the sulfate in a waste stream. The waste stream itself is relatively dilute and poses a pollution problem in disposing of it which is difficult and expensive to handle.
The use of alkanolamines, such as trialkanolamines, has been found to be a highly efficient way of absorbing sulfur dioxide from gases in a cycle in which the alkanolamine solvent contacts the waste gas to absorb the sulfur oxides and is thereafter stripped by heat to release the sulfur dioxide as a gas whereupon it is collected for safe disposal. The stripped alkanolamine is then recycled back to the absorber for further contact with incoming waste gases and further absorption of sulfur oxide. This type of system is disclosed in U.S. Pat. Nos. 3,620,674 and 3,904,735. Heat stable salts, such as those mentioned hereinabove, accumulate in the recycling absorbent to a troublesome extent and must be removed in order to maintain the absorbing capability of the absorbent. The latter patent does disclose a sulfate purge cycle in which a portion of the lean absorbent is treated with potassium hydroxide or potassium carbonate to precipitate out the sulfate as potassium sulfate. While this type of purge system is quite effective in removing sulfates, it is severely limited in removing other heat stable salts or their divalent sulfur oxyanions, which also seem to interfere, however, with the sulfate removal. Furthermore, large amounts of wet sulfates are produced and create a severe disposal problem. There does not appear to be any provision made in U.S. Pat. No. 3,620,674 for removing the heat stable salts and/or their sulfur oxyanions from the absorbent which gradually but inevitably loses effectiveness because of the accumulation of heat stable salts therein.
Anion exchange resins have been used in the past to separate sulfur dioxide from gas mixtures. An example of prior art of this type is U.S. Pat. No. 3,330,621 which utilizes a mass of solid pyridine group-containing particles to contact the sulfur dioxide-containing gas to bind the sulfur dioxide as sulfite groups to the pyridine groups. Thereafter, oxygen is added to oxidize the sulfite groups on the pyridine groups to form sulfate groups. Then, the sulfate groups on the pyridine groups are treated with ammonia to form ammonium sulfate which is then recovered and the pyridine group-containing particles are recycled for re-contact with the SO.sub.2 -containing gases. This type of prior art process involves the use of extremely high quantities of anion exchange resin and excessively large quantities of ammonia and/or other reagents and presents a disposal problem for the large quantities of ammonium sulfate which are produced because the total quantity of sulfur dioxide in the gas is converted via the pyridine group-containing particles into ammonium sulfate.
Anion exchange resins have also been used to treat the total amount of a recyling absorbent, such as sodium hydroxide or ammonium bisulfite. In U.S. Pat. No. 3,896,214, the sulfur dioxide and sulfur trioxide in the gases are washed with sodium hydroxide to convert substantially all the sulfur dioxide and sulfur trioxide content thereof into sodium bisulfite and/or sodium sulfite and sodium sulfate which are then contacted with a hydroxyl-containing weak base or strong base anion exchange resin to substitute the hydroxyl groups on the resin with the bisulfite, sulfite and sulfate anions thereby regenerating the sodium hydroxide. The resulting resin sulfate, sulfite and/or bisulfite is regenerated by treatment with aqueous lime hydrate to form calcium sulfate and calcium sulfite and/or calcium bisulfite and to substitute hydroxyl anions on the resin. The calcium salts are removed as a sludge by dewatering. In U.S. Pat. No. 3,833,710, aqueous ammonium sulfite is used as an absorbent and is converted to aqueous ammonium bisulfite after picking up the sulfur dioxide in the gas. The aqueous ammonium bisulfite solution is contacted with a weak base anion exchange resin in the hydroxyl form to convert the resin to the bisulfite form and regenerate the ammonium sulfite absorbent solution. Both this and U.S. Pat. No. 3,896,214 are based on the removal from the gases of the total amount of the SO.sub.2 content as well as the SO.sub.3 content by utilizing ion exchange. This requires the utilization of extremely large amounts of anion exchange resins which are expensive and also requires the use of extremely large amounts of reagents to regenerate the anion exchange resin which is not only expensive but presents a considerable waste disposal problem for liquid wastes that are relatively quite dilute when consideration is given to the need for washing the resin after each liquid pass during regeneration. Furthermore, the SO.sub.2 content is rendered unavailable for recycle.
U.S. Pat. No. 2,713,077 discloses the use of strong base anion exchange resins to remove carbonyl sulfides from hydrocarbon fluids, such as hydrocarbon gases, produced by the thermal or catalytic cracking of petroleum oils or by the reaction of steam with coke or hydrocarbons. U.S. Pat. No. 3,297,401 removes arsenic and iron contamination from phosphoric acid preparations with a weak base liquid anion exchange resin. In each of these patents the spent anion exchange resin can be regenerated with sodium hydroxide. Neither patent relates to the removal of sulfur dioxide and heat stable salts from gases containing them or their ingredients.
Other prior art processes for removing hydrogen sulfide from gases containing it and recovering sulfur are disclosed in U.S. Pat. No. 3,561,925 and 3,598,529. The reaction of sulfur dioxide and hydrogen sulfide is carried out in a solvent phase in U.S. Pat. No. 3,598,529 and no provision is made for recovering and recycling SO.sub.2. In U.S. Pat. No. 3,561,925 sulfur dioxide is reacted with ammonium sulfide (produced by washing the H.sub.2 S-containing gases with ammonia and water) to form sulfur and regenerate ammonia.
A complex series of chemical reactions is employed in U.S. Pat. No. 3,719,742 to reduce metal pyrosulfites to the corresponding metal sulfides which are converted to the corresponding metal carbonate and hydrogen sulfide. Included, in one embodiment, is the Claus reaction for producing sulfur from the H.sub.2 S formed in the conversion to the metal carbonate and the SO.sub.2 formed during the reduction to the metal sulfide or obtained by burning some of the H.sub.2 S. There is no disclosure, however, of incinerating the tail gas from the Claus reactor to convert traces of H.sub.2 S to SO.sub.2, recovering the SO.sub.2 and recycling it to the Claus reactor.
There are also prior processes for removing sulfur oxide from industrial fumes wherein H.sub.2 S is utilized as a reagent for removing sulfur oxides which are dissolved in solvents such as alkali metal bisulfites, aqueous ammonia or ammonium sulfite. Prior art processes of this type are exemplified by those described in U.S. Pat. Nos. 3,833,710; 3,883,638 and 3,839,549, but they are not concerned with the removal of hydrogen sulfide from gases containing it.